This invention relates to the beneficiation of coal and more particularly to an improved process for the beneficiation of coal and separation of impurities therefrom.
Known resources of coal and other solid carbonaceous fuel materials in the world are far greater than the known resources of petroleum and natural gas combined. Despite this enormous abundance of coal and related solid carbonaceous materials, reliance on these resources, particularly coal, as primary sources of energy, has been for the most part discouraged. The availability of cheaper, cleaner burning, more easily retrievable and transportable fuels, such as petroleum and natural gas, has in the past, cast coal to a largely supporting role in the energy field.
Current world events, however, have forced a new awareness of global energy requirements and of the availability of those resources which will adequately meet these needs. The realization that reserves of petroleum and natural gas are being rapidly depleted in conjunction with skyrocketing petroleum and natural gas prices and the unrest in the regions of the world which contain the largest quantities of these resources, has sparked a new interest in the utilization of solid carbonaceous materials, particularly coal, as primary energy sources.
As a result, enormous efforts are being extended to make coal and related solids carbonaceous materials equivalent or better sources of energy, than petroleum or natural gas. In the case of coal, for example, much of this effort is directed to overcoming the environmental problems associated with its production, transportation and combustion. For example, health and safety hazards associated with coal mining have been significantly reduced with the onset of new legislation governing coal mining. Furthermore, numerous techniques have been explored and developed to make coal cleaner burning, more suitable for burning and more readily transportable.
Gasification and liquefaction of coal are two such known techniques. Detailed descriptions of various coal gasification and liquefaction processes may be found, for example, in the Encyclopedia of Chemical Technology, Kirk-Othmer, Third Edition (1980) Volume 11, pages 410-422 and 449-473. Typically, these techniques, however, require high energy input, as well as the utilization of high temperature and high pressure equipment, thereby reducing their widespread feasibility and value.
Processes to make coal more readily liquefiable have also been developed. One such process is disclosed in U.S. Pat. No. 4,033,852 (Horowitz, et al). This process involves chemically modifying a portion of the surface of the coal in a solvent media, the effect of which renders the coal more readily liquefiable in a solvent than natural forms of coal thereby permitting recovery of a liquefiable viscous product by extraction.
In addition to gasification and liquefaction, other methods for converting coal to more convenient forms for burning and transporting are also known. For example, the preparation of coal-oil and coal-aqueous mixtures are described in the literature. Such liquid coal mixtures offer considerable advantages. In addition to being more readily transportable than dry solid coal, they are more easily storable, and less subject to the risks of explosion by spontaneous ignition. Moreover, providing coal in a fluid form makes it feasible for burning in conventional apparatus used for burning fuel oil. Such a capability can greatly faciliate the transition from fuel oil to coal as a primary energy source. Typical coal-oil and coal-aqueous mixtures and their preparation are disclosed in U.S. Pat. No. 3,762,887, U.S. Pat. No. 3,617,095, U.S. Pat. No. 4,217,109, U.S. Pat. No. 4,101,293 and British Pat. No. 1,523,193.
Regardless, however, of the form in which the coal is ultimately employed, the coal or coal combustion products must be cleaned because they contain substantial amounts of sulfur, nitrogen compounds and mineral matter, including significant quantities of metal impurities. During combustion, these materials enter the environment as sulfur dioxides, nitrogen oxides and compounds of metal impurities. If coal is to be accepted as a primary energy source, it must be cleaned to prevent pollution of the environment either by cleaning the combustion products of the coal or the coal prior to burning.
Accordingly, physical as well as chemical coal cleaning (beneficiation) processes have been explored. In general, physical coal cleaning processes involve pulverizing the coal to release the impurities, wherein the fineness of the coal generally governs the degree to which the impurities are released. However, because the costs of preparing the coal rise exponentially with the amount of fines to be treated, there is an economic optimum in size reduction. Moreover, grinding coal even to extremely fine sizes may not be effective in removing all the impurities. Based on the physical properties that effect the separation of the coal from the impurities, physical coal cleaning methods are generally divided into four categories: gravity, flotation, magnetic and electrical methods. In contrast to physical coal cleaning, chemical coal cleaning techniques are in a very early stage of development. Known chemical coal cleaning techniques include, for example, oxidative desulfurization of coal (sulfur is converted to a water-soluble form by air oxidation), ferric salt leaching (oxidation of pyritic sulfur with ferric sulfate), and hydrogen peroxide-sulfuric acid leaching. Other methods are also disclosed in the abovenoted reference to the Encyclopedia of Chemical Technology, Volume 6, pages 314-322.
While it is obvious from the foregoing that enormous efforts have been made to make coal a more utilizable source of energy, further work and improvements are necessary and desirable before coal and coal admixtures are accepted on a wide scale.